The present invention is related to disc drives. In particular, the present invention is related to disc drive servo control systems.
In a computer disc drive, data is stored on discs in concentric tracks. In disc drives with relatively high track densities, a servo feedback loop is used to maintain a head over the desired track during read or write operations. This is accomplished utilizing prerecorded servo information either on a dedicated servo disc or on sectors that are interspersed along a disc. During track following, the servo information sensed by the head is demodulated to generate a position error signal (PES) which provides an indication of the distance between the head and the track center. The PES is then converted into an actuator control signal, which is used to control an actuator that positions the head.
Historically, only one actuator, typically a voice coil motor (VCM), was used to position the head. Recently, micro-actuators have been proposed that would be used in combination with the VCM to position the head. Because they are small, such micro-actuators generally have a better frequency response than the VCM. As such, they are better able to follow high frequency control signals.
Although the micro-actuators have a better frequency response than voice coil motors, they also have a more limited range of motion. When a micro-actuator reaches the limit of its range of motion, increases in the control signal to the micro-actuator do not result in any more movement of the micro-actuator. Under these conditions, micro-actuator is said to be saturated.
When a micro-actuator saturates in a servo loop, it causes two problems. First, because the stability of the servo loop is designed based on the inclusion of a responsive micro-actuator, when the micro-actuator saturates, it can cause the entire servo loop to become unstable. Second, the actuator controller corresponding to the micro-actuator typically includes some form of past state feedback. When the micro-actuator is unable to move to the position set by the controller, the past state stored in the controller is no longer tied to the movement of the head. This discrepancy in the past states affects the performance of the controller even after the input to the controller drops below a value that would cause the micro-actuator to saturate. The errors caused by past state discrepancy are known as windup.
Several techniques have been used in control system design to avoid instability and windup resulting from actuator saturation. However, most of these schemes are designed for single-loop systems, which are not directly applicable to dual-stage anti-windup control systems. One technique applicable to dual actuator disc drive servo systems utilizes the micro-actuator feedback signal to avoid destabilization. This technique is complex and hinders tracking performance of the servo loop.
The present invention addresses these problems and offers other advantages over the prior art.
The present embodiments relate to servo systems that employ an adaptive anti-windup scheme that adjusts the gain of the VCM and micro-actuator controllers to avoid micro-actuator saturation under external vibration disturbances, thereby addressing the above-mentioned problems.
One embodiment relates to an apparatus for positioning a head over a disc in a disc drive while maintaining servo loop stability. The apparatus includes an actuator-head assembly having a large-scale actuator and a micro-actuator that are both able to move the head over the disc. A saturation adjustment component detects when a micro-actuator controller is producing a micro-actuator control value that will saturate the micro-actuator. Using the micro-actuator control value, the saturation adjustment component generates a saturation error value. An adaptive anti-windup circuit transfers control to the large-scale actuator when the saturation error value is generated by the saturation adjustment component.
Another embodiment relates to a method of maintaining stability in a servo loop of a disc drive that has a large-scale actuator, a micro-actuator, a head, and a disc. The method includes generating a position value based on the position of the head over the disc and generating a micro-actuator control value for driving the micro-actuator based on a position error value. A saturation error value is generated based on the amount by which the micro-actuator control value exceeds a threshold saturation value. Control is transferred to the large-scale actuator when the saturation error value is generated.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.